1. Field of the Invention
This invention relates to electronic digital to analog converter circuits suitable for converting a digital signal to an analog signal, and analog to digital converter circuits capable of converting an analog signal to a digital representation of that analog signal.
2. Description of the Prior Art
Digital to analog converters and analog to digital converters are well known in the prior art. For example, one such digital to analog converter is disclosed in U.S. Pat. No. 4,384,274, which issued May 17, 1983 and which is assigned to American Microsystems, Inc., the assignee of this application. U.S. Pat. No. 4,384,274 is hereby incorporated by reference. The analog output signal of the digital to analog converter of this prior art circuit is unipolar in that the analog signal generated is always positive. Similarly, in the analog to digital converter disclosed in U.S. Pat. No. 4,384,274, the analog voltage to be converted must be of a single polarity. Such unipolar digital to analog and analog to digital converters cannot be used when it is desired to provide a bipolar analog output signal from a digital to analog converter, or when it is desired to provide a digital representation of a bipolar input signal applied to an analog to digital converter.
One attempt at solving this problem is disclosed in U.S. patent application Ser. No. 06/169,980, filed July 18, 1980, assigned to American Microsystems, Inc., and hereby incorporated by reference. This prior art invention uses a single reference voltage for comparing both positive and negative input signals applied to the analog to digital converter. This circuit utilizes a capacitor array and a plurality of three-way switches in order to convert a bipolar analog input voltage to a digital representation thereof. The use of a capacitor array generally requires a large surface area when the digital to analog or analog to digital converter is fabricated as a single integrated circuit semiconductor device, thus resulting in a rather large and expensive device compared to most linear integrated circuits.